The present invention relates generally to a novel hollow metallic baseball bat, and more particularly, is directed to the method and apparatus resulting therefrom for modifying the interior air pressure of the bat.
It is presently common practice to fabricate baseball bats of solid wood or hollow or filled aluminum construction. The hollow aluminum bats are conventionally formed and shaped of aluminum tubing of suitable wall thickness with the handle end enclosed by a formed aluminum end cap or knob which is secured to the handle construction. The large end or ball striking end of the bat is closed by metal spinning or by a solid rubber plug which is cemented in place. The end plug acts to provide necessary balance weight and closure to produce an entirely satisfactory device. One such bat construction is disclosed in U.S. Pat. No. 3,479,030 to Merola.
In accordance with the present invention, it has been found that the efficiency and capability of a hollow aluminum bat of conventional design can be greatly improved by varying the internal air pressure to achieve optimum results. The improved ball driving characteristics of the modified internal pressure bat have as their basis certain fundamental results which can be explained by some of the concepts of theory of mechanical vibrations.
In accordance with fundamental principles of mechanical vibrations, when a body is set into motion, if the motion is periodic, that is, if it repeats itself with time, this motion is termed vibration. The number of independent variables used to completely specify the configuration of the vibrating system is referred to as the number of degrees of freedom of the system. The response of a body to a disturbance may be analyzed into a number of periodic motions, one periodic motion for each degree of freedom of the body. These periodic motions are called the normal modes of free vibration and each of these normal modes has an associated natural frequency of free vibration.
In order to study the vibratory properties of a body, a knowledge of the free vibration characteristics of the body is important. In particular, in order to predict the response of a body to any external excitation, the natural frequencies of free vibration together with their associated normal modes of vibration must be known. This is due to the fact that the vibratory response of a body due to some disturbing force is composed of a linear combination, or a weighted sum, of all of the individual normal modes of vibration.
In addition, it is important to know the free vibration characteristics of a body in order to know or to calculate the excitation frequencies which would give rise to the phenonmenum called resonance. A system is considered to be in resonance when the exciting force oscillates with a frequency that is the same or nearly the same as one of the natural frequencies of the system, whereby the addition of the frequencies results in a response of the system that becomes very large. The critical structural frequency of a bat as used herein is defined as the resonance condition produced when natural frequencies are modified by modifying the internal air pressure to produce maximum ball striking force.
It should be noted here that it is this phenonmenum of resonance that provides the basis of the improved performance of the modified internal pressure bat of the present invention.
With particular reference to the improved bat of the present invention, a hollow aluminum bat consists of an infinite number of particles all coupled together elastically. The bat therefore has an infinite number of degrees of freedom and correspondingly, an infinite number of natural frequencies and modes of vibration. In the context of the present invention, only the lowest frequencies, at most about twenty of these frequencies, are of any practical importance in predicting the response due to an excitation.
In order that the vibration theory above set forth be put to use in describing the highly improved performance of a modified internal pressure bat, the actual mechanics of a bat striking ball and the nature of the response resulting from this motion must first be considered.
In the process of striking a ball with a bat, the batter produces within the bat a vibratory motion. These vibrations are due to an excitation produced by the motion of the bat during the batter's swing and the short duration force or impulse on the bat resulting from the ball making contact with the bat. Along with the energy produced by the batter's swing, this vibratory motion in the bat imparts a force to the ball while the ball is in contact with the bat. The obvious result of this contact is the placement of the ball in flight.
When the bat being used is of the conventional hollow aluminum type with a sealed in place end rubber plug, the vibratory motion produced within the bat is of a frequency much different in magnitude than any of the important natural frequencies of the bat. Because of this difference, resonance is not approached and the force imparted to the ball due to the vibration of the bat is of a nearly negligible amount.
When as in the present invention, the pressure maintained within the hollow aluminum bat is modified, an important effect takes place, namely, significant changes in the natural frequencies and normal modes of vibration of the bat can be noted. The changes in the natural frequencies and the normal modes of vibration of the bat are due to the displaced configuration and the resulting change in the bat's stiffness characteristics which occur due to the modification of the pressure within the hollow chamber or interior of the bat.